diff --git a/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java b/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
index 9209bd3135099..87d004040c3a0 100644
--- a/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
+++ b/common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java
@@ -642,9 +642,13 @@ public UTF8String substring(final int start, final int until) {
     }
 
     int j = i;
-    while (i < numBytes && c < until) {
-      i += numBytesForFirstByte(getByte(i));
-      c += 1;
+    if (until == Integer.MAX_VALUE) {
+      i = numBytes;
+    } else {
+      while (i < numBytes && c < until) {
+        i += numBytesForFirstByte(getByte(i));
+        c += 1;
+      }
     }
 
     if (i > j) {
@@ -663,9 +667,8 @@ public UTF8String substringSQL(int pos, int length) {
     // refers to element i-1 in the sequence. If a start index i is less than 0, it refers
     // to the -ith element before the end of the sequence. If a start index i is 0, it
     // refers to the first element.
-    int len = numChars();
     // `len + pos` does not overflow as `len >= 0`.
-    int start = (pos > 0) ? pos -1 : ((pos < 0) ? len + pos : 0);
+    int start = (pos > 0) ? pos -1 : ((pos < 0) ? numChars() + pos : 0);
 
     int end;
     if ((long) start + length > Integer.MAX_VALUE) {